[3.10] bpo-45494: Fix parser crash when reporting errors involving invalid continuation characters (GH-28993) (GH-29070)

There are two errors that this commit fixes:

* The parser was not correctly computing the offset and the string
  source for E_LINECONT errors due to the incorrect usage of strtok().
* The parser was not correctly unwinding the call stack when a tokenizer
  exception happened in rules involving optionals ('?', [...]) as we
  always make them return valid results by using the comma operator. We
  need to check first if we don't have an error before continuing..
(cherry picked from commit a106343f632a99c8ebb0136fa140cf189b4a6a57)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/Parser/pegen.c b/Parser/pegen.c
index 1bb975d..66e4b19 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -371,10 +371,18 @@ tokenizer_error(Parser *p)
             errtype = PyExc_IndentationError;
             msg = "too many levels of indentation";
             break;
-        case E_LINECONT:
-            col_offset = strlen(strtok(p->tok->buf, "\n")) - 1;
+        case E_LINECONT: {
+            char* loc = strrchr(p->tok->buf, '\n');
+            const char* last_char = p->tok->cur - 1;
+            if (loc != NULL && loc != last_char) {
+                col_offset = p->tok->cur - loc - 1;
+                p->tok->buf = loc;
+            } else {
+                col_offset = last_char - p->tok->buf - 1;
+            }
             msg = "unexpected character after line continuation character";
             break;
+        }
         default:
             msg = "unknown parsing error";
     }